When images of the entrance of a building such as a retail store are shot with a surveillance camera or the like, incidence of light from multiple light sources (e.g., incidence of external light and illuminating light in the building) causes a problem in that the color tone changes greatly from position to position even in images shot by the same camera due to the influence of the light. If the color tone changes from region to region in an image, it becomes difficult to make a judgment on a particular color of clothing (as one of the features of a person), the identity of a person, etc. based on images shot with a surveillance camera or the like.
Examples of a method for correcting the color tone of an image include a method of performing the color correction by using a color chart on which a plurality of colors as the reference (natural colors, chromatic colors, primary colors, gray scale, etc.) are arranged. According to this method, the color tone of an image shot with a camera can be corrected by calculating color correction parameters for the image based on the result of shooting the color chart.
There are also methods of performing the color correction by using a brightness transfer function (e.g., Non-patent Literatures 1 and 2). The Non-patent Literature 1 describes examples of the brightness transfer function for making the object association (object re-identification) between cameras differing in the color tone. For example, the color of the clothing of a person A, wearing apparently blue clothing in an image shot with a camera A, can be recognized as light blue in an image shot with a camera B due to a change in the color tone. This method makes it possible to determine the identity of objects in images shot by different cameras even when the cameras differ in the color tone, by associating image feature values of the blue clothing and the light blue clothing with each other.
Non-patent Literature 2 describes a brightness transfer function designed by taking light source fluctuation into consideration.